Make the pairs of lines match up by making each second line same as first:
# Appending text:
The name "Vim" is an acronym for "Vi IMproved"
The name "Vim" is an acronym for
# Editing text:
Vim is a text editor originally released by Bram Moolenaar in 1991 for the Amiga
Trivia: Vim is a text editor released by Bram Moolenaar in 1991 for the Amiga
# Deleting text:
Vim has a vi compatibility mode
Vim has a vi compatibility mode but when not in this mode Vim has many enhancements over vi

to this:

Make the pairs of lines match up by making each second line same as first:

# Appending text:
The name "Vim" is an acronym for "Vi IMproved"
The name "Vim" is an acronym for "Vi IMproved"
# Editing text:
Vim is a text editor originally released by Bram Moolenaar in 1991 for the Amiga
Vim is a text editor originally released by Bram Moolenaar in 1991 for the Amiga
# Deleting text:
Vim has a vi compatibility mode
Vim has a vi compatibility mode

Nikodemus answers my follow-on question correctly. I just want to note that in vi-mode ':help :t' gives you the t-to-co equivalence, while ':help t' gives you something about a "till" command. The extra colon is everything.
–
Bruce EdigerDec 13 '11 at 19:57

That's exactly what I don't understand. So t which is the same thing as co, here is used with range 1 and address .?
–
AliDec 13 '11 at 20:04

1

Well ... the range is 1 unless specified different. +1 is the position, 1 after the /#/. That line is copied to after the position given in the argument, which is .
–
NikodemusDec 13 '11 at 22:43